the Arduino Nano BLE 33 has a other chip typ then the normal nano. The SoftwareSerial is a function for the normal one. A way to do it with the nano BLE is:
#include "wiring_private.h"
Uart mySoftwareSerial(&sercom0, 4, 5, SERCOM_RX_PAD_1, UART_TX_PAD_0);
[...]
void setup(){
pinPeripheral(4, PIO_SERCOM_ALT);
pinPeripheral(5, PIO_SERCOM_ALT);
mySoftwareSerial.begin(9600);
[...]